Radiotherapy works by directing a beam of harmful radiation towards the site of (for example) a tumour. The radiation inflicts damage on the tumour and causes its reduction. In order to prevent collateral damage to the healthy tissues surrounding the tumour, the beam will be shaped to reduce the dose applied outside the tumour, for example by conforming to the outside shape of the tumour. It will also (generally) be directed towards the tumour from a variety of different directions along axes that are centred on the tumour. Thus, by rotating the source around the patient and varying the shape of the tumour, a three-dimensional dose distribution can be built up which is at a maximum within the tumour and is minimised elsewhere.
It is therefore important to ensure that the patient is correctly positioned within the apparatus. The apparatus will work to its own set of co-ordinate axes and will expect the tumour to be positioned in the correct location at its “isocentre”, the point in space about which the radiation source rotates and which is therefore always on the radiation axis. Fine control of the patient position can be achieved by providing an articulated couch for the patient, and modern couches are able to perform adjustments to the patient position in all six degrees of freedom. This is controlled in response to data obtained from a diagnostic x-ray source integrated with the radiotherapeutic apparatus, which can provide real time information as to the current position of the patient.
However, it is necessary to ensure that the patient position is approximately correct before such apparatus can be used to fine-tune the position of the patient. The initial positioning of the patient to an accuracy of a few millimeters is therefore assisted by providing a light source within the apparatus, together with one or more mirrors (as necessary) to direct the light beam along the path of the radiation. Cross-hairs within the beam path can be used to align the patient, and the light field can be used to check operation of the various collimators that are provided in order to limit the shape of the radiation beam.
One such collimator is the so-called “multi-leaf collimator” (MLC), as shown (for example) in EP-A-0,314,214. This consists of a plurality of leaves that can be moved into and out of the radiation path; each leaf has a sufficient depth along the radiation axis to absorb the incident radiation, and a narrow width transverse to the radiation axis. A large number of such leaves are placed alongside each other in two opposing banks, and each can be moved independently so that they can (collectively) define an arbitrary edge to the radiation field.
One difficulty that can arise when the MLC is being tested with the optical light source is that the significant depth of the leaves along the beam axis and at a very shallow angle thereto allows them to reflect the incident light. Where a particular leaf is extended or withdrawn significantly beyond its adjacent leaves, such reflective surfaces are created in the beam path and lead to the phenomenon of “ghosting”, whereby a spurious bright line is created in the field. Previous efforts to eliminate ghosting have relied on surface treatment of the leaves to reduce their reflective properties, but the very shallow angle at which they are presented to the light source makes this difficult.